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I have a Fujitsu AOU36RLX that is barely cooling. It is the type with the ceiling coil, not the cassette. It seemed to be working but one day while fooling with the settings on the thermostat, It made a loud hiss when it started and then a bang and the circuit breaker tripped. Later I found out the bang was the wires jumping in the conduit because if a bad short. Inside the unit I found the chips exploded and arcing in the ACTPM module.

I replaced the ACTPM but the unit would still trip the breaker immediately.

I called tech support and they said to replace the ACTPM again plus the main inverter board and the condenser fan motor.

That got it working but barely cooling. Then Tech said to replace the expansion valve coil. That did not help. Then they said the check the charge. The charge was correct.

Then they said to replace the expansion valve because it was stuck. That did not help.

Right now it is moaning like a ghost. The moaning changes frequency up and down. Sometimes the unit will not run at all and you have to "re boot" to get it moaning and barely cooling again.

Does anyone have any ideas Before I call tech support again?

I have installed a separate new system in the same space so the pressure is off to get this one working again. I want to repair it before I move it elsewhere.

I have a list of the resistance of all of the thermistors in the unit and I was thinking of ohming them out.

Then there is the possibility that I plugged one of the myriad of tiny plugs into the main inverter board incorrectly. It is a miserable job because you cannot raise the board enough to see what you are doing because the leads are so short. It is like the motherboard of a computer with those tiny connectors. Standing on a ladder makes it even more difficult. Fujitsu needs to leave more slack in those leads.

However my personal theory is that the logic in the main board would not allow the unit to even start if a sensor was way off. Likewise with the expansion valve. The shaft of the needle is threaded and the core inside of the valve is like a stepper motor. It has five wires leading to the coil and two of them give feedback to the board telling it that the shaft has turned so many degrees. If it did not turn, you would get an error message and the unit would not start.